1. Field of the invention
The present invention relates to an image forming apparatus which forms an image by use of a photosensitive toner.
2. Description of the prior art
In an image forming apparatus, electrophotography or electrostatic printing is generally used for the formation of images. Electrophotography utilizes an image-bearing member in the form of a drum or a belt which is coated with a photoconductive layer, while electrostatic printing utilizes an image-bearing member in the form of a drum or a belt which is coated with a dielectric layer. In either case, as the image-bearing member rotates, an electrostatic latent image is first formed on the image-bearing member and then developed with a dry developer (i.e., toner) into a toner image. The toner image thus formed on the image-bearing member is then transferred onto a sheet of copy paper, resulting in a final image thereon.
In such an image forming apparatus, around the image-bearing member in the form of a drum or a belt, a charging means, an exposure means, a developing means, a transfer means and a cleaning means are disposed in that order in the direction of the rotation of the image-bearing member. Since it is desirable to make the image forming apparatus as small as possible, each means disposed in the apparatus is also required to be smaller in size.
After the formation of an image, toner remaining on the image-bearing member is removed by the cleaning means. In general, the used toner removed by the cleaning means is delivered into a used toner container to be stored therein. Another type of image forming apparatus has been proposed in which the used toner removed by the cleaning means is fed back to the developing means and used again for the subsequent image forming processes. Such an image forming apparatus can be reduced in size to some degree because it does not require the used toner container, and furthermore, the reuse of the toner decreases the operating cost of the apparatus.
However, the size of an image forming apparatus of the above-mentioned type cannot be sufficiently reduced for the following reason: Between the developing means and the cleaning means, the charging means, the transfer means, and the like, are located. Thus, the developing means and the cleaning means cannot be formed as a single unit, so that a separate transporting system is required for transporting the used toner from the cleaning means into the developing means. Since such a used toner-transporting system should be mounted in the image forming apparatus, the whole apparatus cannot be sufficiently reduced in size.
Another image forming apparatus has been developed in which a magnetic brush or a fur brush functioning as a developing means for supplying toner onto the image-bearing member also functions as a cleaning means for removing the used toner from the image-bearing member. In such an apparatus, the magnetic brush or the fur brush first supplies toner onto the electrostatic latent image on the image-bearing member to develop it into a toner image, which is then transferred onto a copy sheet. Thereafter, the toner remaining on the image-bearing member is removed by the magnetic brush or the fur brush. Such an image forming apparatus does not require a separate cleaning means or a used toner-transporting system, so that the apparatus can be greatly reduced in size. However, since the developing means (magnetic brush or fur brush) also functions as the cleaning means, the development of the latent image on the image-bearing member cannot be simultaneously performed with the cleaning of the image-bearing member. For example, with the image-bearing member in the form of a drum, the development of the latent image is performed as the image-bearing member completes its first revolution, and then the image-bearing member is cleaned of the remaining toner during its second revolution. Thus, the image-forming processes cannot be continuously performed, so that the copy speed is reduced.
Still another image forming apparatus has been developed in which a blade which can move to, and away from, the image-bearing member is located upstream (viewed from the rotation of the image-bearing member) of the developing means. In this type of image forming apparatus, the blade is kept away from the image-bearing member during the process of developing the latent image thereon, while the blade is kept in contact with the image-bearing member during the process of cleaning it of the remaining toner. Thus, also in this image forming apparatus, the developing and cleaning processes cannot be performed simultaneously. Thus, the image-forming processes cannot be continuously performed, resulting in a reduced copy speed. Furthermore, in such an apparatus, the blade must be accurately controlled to move toward, and away from, the image-bearing member in synchronization with the rotation of the image-bearing member. Thus, the apparatus requires a complicated system for controlling the movement of the blade. The apparatus is also disadvantageous in that the image-bearing member tends to be damaged by the blade because the blade repeatedly moves to, and away from, the image-bearing member. This deteriorates the quality of the resultant image.